1. Field of Invention
The present invention relates to a method and apparatus for generating a constant-envelope composite transmission signal and, more particularly, to techniques for efficiently combining more than three information signals into a composite, constant-envelope signal using a combination of majority voting logic and interplex modulation schemes.
2. Description of the Related Art
Combining multiple signals on the same radio frequency (RF) carrier is often desirable in both one-way and two-way communications systems, and the importance of signal combining techniques will grow as RF communications systems continue to proliferate and RF spectrum becomes increasingly crowded. Existing methods of signal combining include techniques that generate composite signals whose instantaneous power varies with time (non-constant-envelope signals), such as linear signal combination. Other existing techniques, such as conventional phase shift keyed/phase modulated (PSK/PM) systems, generate constant-envelope composite signals.
Linear methods that generate non-constant-envelope composite signals result in power-inefficient mechanizations, because the power amplifiers that are used for transmission of the composite signals must operate in the linear region. Power amplifiers are much more efficient when operated in the saturated mode. Therefore, constant-envelope signal structures are required if full-power, undistorted transmission is sought.
For example, in a CDMA cellular telephone system, linear superposition of chip-synchronous, orthogonal signals to be transmitted from a base station is a theoretically lossless multiplex if the subsequent transmission chain remains linear. Maintaining linearity requires a linear high power amplifier (HPA). Since any HPA characteristic eventually saturates as its input power increases, such base station transceiver linear amplifiers are typically run at 4–5 dB average power backoff to accommodate peak power needs. In addition, the rather severe spectral containment filtering applied to each user signal before multiplexing creates amplitude fluctuations of 4–5 dB peak-to-average power, requiring additional backoff. Consequently, total backoff can easily be 9 or 10 dB in this particular context.
Thus, linear combination techniques are maximally efficient in the sense that there is no actual signal power loss, but the overall efficiency of such techniques is compromised by the need to operate the amplifier at a significant power back-off to accommodate the instantaneous signal envelope fluctuations. Further, conventional PSK/PM systems have limited power efficiency, because PSK/PM systems include unmodulated carrier and cross modulation terms, which represent wasted power.
An alternative approach to producing greater average power is to achieve a more effective allocation of the loss budget between the multiplexer and the high power amplifier. Applied to orthogonal waveforms, non-linear multiplex methods that produce a constant-envelope composite signal permit a greater fraction of the available transmitter power to be used for communication, but at the expense of a multiplexing loss that may be characterized as either cross-talk (induced non-orthogonality or harmonic distortion) or receiver cross-correlation mismatch. This multiplexing loss, however, is typically smaller than the power backoff it replaces, resulting in a favorable trade.
Constant-envelope composite signals would be particularly beneficial in a number of presently evolving systems. For example, the Global Positioning System (GPS) is an application in which constant-envelope composite signals would be beneficial. This system includes a constellation of Earth-orbiting satellites that transmit signals useful for determining position. By measuring the time delay in broadcasted signals received from several of these satellites, a receiver can determine its own position using trilateration. Continually evolving GPS system requirements necessitate the simultaneous transmission of multiple signals from each of the GPS satellites, making constant-envelope signals of great interest in developing future GPS signal structures and system architectures.
Under GPS modernization programs, the U.S. government is studying techniques to enhance both the military and civilian utility of GPS. A possible outcome of this effort is the inclusion of three or four distinct PN codes in the signal transmitted by a satellite at one frequency. As military and civilian requirements for GPS change over time, operational modifications will continue to be necessary. Critical signaling parameters, such as chip rates, code types, fixed carrier offset, hopping sequences for hopped carrier offset, and relative power ratios, may require modification throughout the operational life of a satellite. In addition to having the capability to produce constant-envelope composite signals, the waveform generator onboard each GPS satellite must be remotely reprogrammable to support generation of a variety of possible future signaling waveforms.
Code division multiple access (CDMA) based cellular telephony and data networks are among other applications for which constant-envelope composite signals would be useful. CDMA transmission of voice/data in terrestrial cellular networks places more stringent requirements on CDMA than any prior applications. Traffic is two-way, and the number of codes per cell can presently be as many as sixty-four and may increase to 128 in the future. Code channels have various functions: pilot, paging, synchronization, control, and traffic. To avoid the dominance of one or a few signals (the “near-far” problem), power control is required at both the subscriber and base station terminals. Because the user mix continually varies due to newly initiated and recently completed calls, user motion and cell-to-cell handoffs, power control is dynamic and rapid (on the scale of milliseconds). Difficult channel conditions are posed by multipath interference and signal obstruction in urban environments. Data rates and traffic loads are certain to increase far beyond present levels. Security of data flowing through the network is needed for operations, maintenance, accurate billing and privacy. Although the primary function of the system is data transmission, there are a variety of reasons, e.g. E911, why determination of subscriber position will be a required, integrated function for all future mobile networks. This complex environment presents an unprecedented need to multiplex CDMA signals efficiently into a constant-envelope signal.
Interplex Modulation and Majority Voting Logic are two techniques that have recently gained consideration for generating constant-envelope, phase modulated composite signals that offer improved efficiency over standard PSK/PM systems. The concept of Interplex Modulation is described by S. Butman et al, in “Interplex—An Efficient Multichannel PSK/PM Telemetry System,” IEEE Transactions on Communications, June 1972, incorporated herein by reference in its entirety. The use of Majority Voting to combine signals is described by J. Spilker et al. in “Code Multiplexing via Majority Logic for GPS Modernization”, Proceedings of the Institute of Navigation (ION) GPS—98, Sept. 15–18, 1998 and by Spilker in “Digital Communications by Satellite”, pp. 600–603, Prentice-Hall, Inc., 1977, both incorporated herein by reference in their entireties.
The composite signal formed by interplex modulation has a constant envelope, i.e., its instantaneous power level does not change with time. Using quadrature carriers, interplex modulation can combine any number of data-bearing, PN spread binary codes and offers a significant improvement in power efficiency over PSK/PM. The component signals may have any assigned power distribution. However, depending upon the desired power ratios among the signal components, the resultant efficiency of Interplex Modulation can degrade rapidly as the number of signals in the mix is increased. Interplex modulation is quite efficient in representing three components (efficiency is never less than 75% for any power allocation), but its efficiency drops rapidly as more signals are added, and is generally not useful for more than five components.
Majority Voting was conceived as a technique to combine multiple signals onto a single RF quadrature component, but has been adapted successfully to general RF modulation. It too represents a significant improvement over PSK/PM and is convenient for large numbers of component signals, but the resultant efficiency can be substantially reduced for scenarios that require relatively large differences in power levels among the various signal components.
Accordingly, there remains a need for a system capable of more efficiently combining signals into a constant-envelope composite signal, particularly where the signals have significantly different power levels or the number of signals to be combined exceeds three.